The present invention relates to a semiconductor laser device which controls an injection current by creating a depletion layer at least in an activation layer.
In the past, a light of a semiconductor laser has been controlled by directly controlling an injection current to the semiconductor laser.
FIG. 1 shows a sectional view and a drive circuit of a prior art semiconductor laser device. The semiconductor laser device is formed by laminating a clad layer 2 of semiconductor laser device wherein a laser beam, on a semiconductor laser substrate 1 made of GaAs, laminating thereon a GaAs activation layer 3 which emits a semiconductor laser beam, laminating thereon a clad layer 4 having the same function as that of the clad layer 2, laminating thereon an insulation layer 5 made of silicon oxide or silicon nitride which pinches an electric current, and finally forming electrodes 6 and 7 at the opposite ends. The GaAs layer 3 is formed between the GaAs layer 1, A12GaAs layer 2 and the AlGaAs layer 4, insulation layer 5.
The driving of the semiconductor laser device thus constructed is controlled by an external current source 8. An electro-optical signal conversion apparatus which is simple in circuit configuration and is easy to modulate like the semiconductor laser device is a very important device. FIG. 2 shows the relationship between an injection current I and a laser beam output L when the semiconductor laser device is driven. As seen from FIG. 2, the semiconductor laser device does not generate the laser beam until the current I exceeds a predetermined level Ith, when it starts to oscillate. The laser beam output of the semiconductor laser device is usually modulated by the injection current. The semiconductor laser device is used for light communication and recording by controlling the current I within a region .DELTA.I shown in FIG. 2. In recent years, the demand for high speed modulation has increased as a result of high density communication and high speed recording. However, where the semiconductor laser device is controlled by an external current as shown in FIG. 1, delay is involved and the voltage drop is large.
Accordingly, a device in which a field effect transistor (FET) and a semiconductor laser device are integrated as shown in FIG. 3 has been proposed. The device comprises a high resistance GaAs substrate 21, a semiconductor laser 22, an FET source 23, a gate 24 to which a modulation signal is applied, a drain 25 and one electrode 26 of the laser. In this device, the modulation signal is applied to the gate 24 to control a laser injection current flowing from the drain 25 to the electrode 26 to modulate the laser. Since the modulation signal applied to the gate 24 is smaller than the injection current in the laser 22, the problems of large signal delay and large voltage drop are resolved.
However, the semiconductor laser device shown in FIG. 3 requires the steps of etching of the substrate 21 and the formation of the source 23, gate 24 and drain 25 of the FET after the formation of the laser device 22, and there are problems in the connection of the laser device and the FET.